In a typical multi-user MIMO communication system, a multi-antenna array in a base station sends multiple data streams selectively and simultaneously to autonomous single-antenna terminals, also referred to as “users,” thereby achieving throughput gains relative to a set of single-antenna links. Multi-user systems of this type are sometimes referred to as “broadcast” MIMO systems. The converse to broadcast MIMO is sometimes referred to as “multiple access” MIMO, and it entails the autonomous single-antenna terminals sending multiple data streams simultaneously to the multi-antenna array in the base station.
One drawback of multi-user MIMO systems is that the base station has to know the propagation characteristics of the forward channel. The process through which the base station obtains this information is generally referred to as training. See, for example, U.S. Patent Application Publication No. 2005/0265290 to Hochwald et al. entitled “Feedback Method for Channel State Information of a Wireless Link,” which is commonly assigned herewith and incorporated by reference herein. Each of the single-antenna terminals may generate forward channel state information in the form of a corresponding channel vector which characterizes the channel between the base station and that terminal. The channel vectors may be based on measurements made by the terminals using pilot signals transmitted by the base station over the forward channel. The terminals transmit their respective channel vectors back to the base station over the reverse channel. These channel vectors collectively form what is referred to as a forward channel matrix.
The base station utilizes the forward channel state information to perform scheduling operations such as, for example, determining which of the terminals will be served in a given time slot or other scheduling interval.
It is well known that the acquisition of forward channel state information by the base station can be considerably facilitated through the use of time-division duplex (TDD) operation. In the TDD context, the principle of reciprocity implies that the reverse channel matrix is equal to the transpose of the forward channel matrix, so the base station can readily obtain the required forward channel state information by simply processing pilot signals transmitted by the terminals over the reverse channel. Thus, TDD operation avoids the need for the terminals to generate channel vectors and transmit such channel vectors back to the base station.
However, in frequency division duplex (FDD) operation, the principle of reciprocity generally does not apply, and the generation and transmission of the above-noted channel vectors remains a requirement. The amount of overhead involved may be prohibitive, especially when the number of terminals is large, or when the channel characteristics are changing rapidly due to terminal mobility.
Accordingly, a need exists for techniques that can reduce the overhead burden associated with estimation of forward channel state information in a multi-user MIMO system, particularly one operating in an FDD mode.